1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of Related Art
In order to improve output characteristics of a nonaqueous electrolyte secondary battery in a low state of charge (hereinafter, referred to as “SOC”), various studies have been made. For example, Japanese Patent Application Publication No. 2008-235150 (JP 2008-235150 A) discloses a nonaqueous electrolyte secondary battery in which a positive electrode active material contains: a lithium-containing metal oxide containing at least Co; and LibFePO4 (wherein b satisfies a condition of 0≤b<1).
According to JP 2008-235150 A, for example, the positive electrode active material such as LiNi0.80Co0.15Al0.05O2 is mixed with 10 mass % or less of lithium iron phosphate having a lower action potential than LiNi0.80Co0.15Al0.05O2. As a result, an abrupt increase in the resistance of the positive electrode active material at a late stage of discharge (that is, low SOC) can be suppressed, and high output can be obtained over a wide range of SOC. However, in consideration of the behavior of a battery during overcharge, there is room for further improvement in such a technique.
For example, as a countermeasure against the overcharge of a battery, a large-sized battery for a vehicle may include a pressure-operated current interrupt device (hereinafter, referred to as “CID”) that physically interrupts a conductive path when an internal pressure of the battery during overcharge exceeds a predetermined pressure (also referred to as “working pressure”). In this case, a positive electrode is required to have a function of causing a reaction with a gas producing agent (also referred to as “overcharge additive”), which is contained in an electrolytic solution or the like, to produce gas such that the CID is rapidly operated during overcharge.
However, in JP 2008-235150 A, lithium iron phosphate which is used as the positive electrode active material having a low action potential has significantly low conductivity. Therefore, this low conductivity is compensated for by coating surfaces of lithium iron phosphate particles with carbon. Further, in order to secure a predetermined battery output, it is necessary to increase a ratio of a conductive material in a positive electrode mixture into which lithium iron phosphate is mixed. Moreover, lithium iron phosphate has low capacity per volume. Therefore, the positive electrode capacity per volume decreases depending on the mixing amount of lithium iron phosphate. Accordingly, in order to maintain the battery capacity, it is necessary to improve the filling factor, that is, the mixture density of a positive electrode mixture layer depending on the mixing amount of lithium iron phosphate.
However, when the mixture density increases, the area of pores in the layer decreases, and the amount of an electrolytic solution which can be stored in the positive electrode mixture layer decreases. As a result, a contact ratio between the positive electrode active material and the gas producing agent contained in the electrolytic solution decreases, and the amount of gas produced during overcharge decreases. Further, there is a possibility in that the internal pressure cannot be efficiently increased due to a decrease in the area of a discharge path for the produced gas.